1. Field of the Invention
This invention relates to a method of forming a rotor for a reluctance machine and particularly, but not exclusively, a rotor for a synchronous reluctance machine.
2. Description of Related Art
A typical prior art synchronous reluctance machine is shown schematically in cross section in FIG. 1. The motor consists of a stator 10, onto which are wound three-phase, sinusoidally distributed, windings 20, 30, 40 in slots. Although only one conductor of each winding is shown in FIG. 1, it will be understood that a practical motor consists of many such conductors distributed in the slots around the stator. Those conductors in adjacent slots, and connected in the same electromagnetic phase, are known as a phase band. The conductors shown in FIG. 1 are each at the center of their respective phase bands.
A two-pole salient rotor 45 is mounted co-axially with the stator 10 on a shaft 48. The stator 10 and rotor 50 are typically constructed from thin laminations of magnetically permeable iron, the planes of lamination being normal to the shaft.
Torque is produced by supplying the three phase windings 20, 30, 40 of the motor of FIG. 1 with sinusoidal currents which have a phase displacement in time, with respect to each other, of 120.degree.. The varying flux generated by the stator 10 causes the rotor poles to move as they attempt to maintain a position of minimum reluctance in the changing magnetic field.
The variation of stator winding current in time may be represented as a rotating phasor. The current in each winding sets up a magnetomotive force (MMF) which is sinusoidally distributed in space around the air gap between the stator 10 and rotor 50 and has a wavelength equivalent to the circumferential distance between the center of each phase band. Thus, a wave of MMF is produced which travels one pole pitch in one half cycle of the supply frequency.
The MMF vector acts on the permeance of the non-uniform air gap to produce a corresponding flux-linkage vector. This flux-linkage vector urges the salient rotor poles into a position of minimum reluctance. It is this tendency of the rotor to align itself with the flux-linkage vector which gives rise to the motor torque which is the output of the rotor.
As indicated by the broken lines D and Q in FIG. 1, there are direct and quadrature axes of the motor, the quadrature axis being 90 electrical degrees from the direct axis. In order to maximize the performance of a synchronous reluctance motor, it is preferable that the difference in the reluctance of the magnetic circuit when the rotor is in the maximum and minimum reluctance positions is as great as possible.
A number of other parameters can be considered in the construction of a synchronous reluctance motor in order to enhance dynamic performance. In particular, the rated output, i.e. the torque that the motor can produce continually for a given temperature rise, should be maximized. Also, it is desirable to limit the amount of torque ripple in the output of the motor. Torque ripple is the variation of output torque as a function of rotor position, which acts upon the rotor inertia causing a corresponding rotor speed variation.
It is known to improve the output of the motor by flux-guiding the rotor. FIG. 2 shows a radial lamination profile of a typical rotor with flux guides 50. The flux guides usually consist of a set of areas of lamination material extending around a segment of the rotor, each flux guide being bounded by a region of relatively low magnetic permeability known as a flux barrier 52. There may be a plurality of such sets of flux guides spaced radially across the rotor, the flux guides in each set being radially separated by flux barriers. It will be seen in FIG. 2 that the flux barriers 52 extend from one rotor pole region 54 into an interpolar region 56. Adjacent flux barriers are separated by a thin inner magnetic bridge 58.
Each flux barrier is terminated in a peripheral magnetic bridge 60. On the one hand, the bridges 58 and 60 should be as thin as possible. The thinner they are, the more limited is the magnetically distorting effect they have on the flux paths defined by the flux guides. The bridges are designed to saturate quickly, and while their magnetic effect as a flux path where one is not wanted is small, it is non-negligible. Ideally, they should not be present at all but they have been considered necessary in the past to keep the lamination in one piece during and after the rotor assembly process. Thus, there is a limit on how thin the bridges can be in order for them to be sufficiently mechanically rigid.
Some stress relief has been effected by forming the radially peripheral ends of the flux barriers with a radius. The prior art rotor lamination of FIG. 2 embodies an acceptance of the need for magnetic bridges in the finished rotor as an essential mechanical feature of the rotor assembly process. Axially laminated rotors are also known, e.g. from U.S. Pat. No. 4,888,513 (Fratta), which is incorporated herein by reference. A typical cross-section of an axially laminated rotor is shown in FIG. 3. The laminations 62, 64, 66 are generally channel-shaped and the laminations lie parallel to the shaft, hence the term `axially laminated`.
An axially laminated rotor may be considered as a flux-guided rotor where the number of sets of flux guides is high, and the radial width of each set of flux guides and associated flux barriers becomes consequentially small.
Each of these techniques has problems. Rotors with a small number of flux guides are relatively simple to produce, since they can be made from punched, radial laminations. In order to provide the required amount of mechanical rigidity, however, and to allow the ferromagnetic flux-guides to be connected to the rotor core, saturating magnetic bridges must be incorporated. These significantly increase the quadrature axis inductance, leading to motor performance degradation.
British Patent No. 1054924, which is incorporated herein by reference, is concerned with providing a method of construction to attempt to overcome the difficulties of the saturating magnetic bridges. It shows a rotor lamination having salient pole portions and circumferential pole portions, initially linked by bridging sections. The spaces between the portions are filled with conducting metal, forming a cage which assists in holding the portions together. However, this cage has an adverse effect on the losses of the rotor. The bridging sections are then removed by milling to avoid any increase in quadrature axis inductance, but this is both a difficult and time consuming exercise.
The milling operation to remove the bridges is also intrusive. In many applications the rotor is designed to rotate at a rate at which the windage on a non-circular section can become significant. Thus, the recesses left after the milling operation are often filled in with a filler material, such as a curable resin. However, the radial forces imposed on the filler are high. The filler is only able to rely on its adhesion to the metal of the rotor to stop it being forced out at speed.
Axially laminated rotors are seldom used because of the extreme difficulty of manufacture and of ensuring stability of the assembly during the lifetime of the machine. It will be seen from FIG. 3 that each lamination has a geometry different from the adjacent ones. This implies a very involved manufacturing process. Then there is the difficulty of securing each bundle of laminations to the central rotor core in a stable and secure way. Although there are several methods of doing this, all are complex and expensive. Even when the rotor is complete, it is usually limited in top speed and dynamic performance because of its construction.